In recent years, as the integration density of a semiconductor device represented by a DRAM increases more and more, the feature size of a pattern to be formed on the semiconductor device shrinks more and more. Under these circumstances, in a semiconductor exposure apparatus, the NA (Numerical Aperture) of a projection optical system increases, and the depth of focus decreases accordingly. Hence, a technique for accurately positioning a reticle (original) and a wafer (substrate) at conjugate positions with respect to the projection optical system has become significant.
The wafer is positioned on the focus (focal point) of the projection optical system by adjusting the positional relationship between the projection optical system and a reference member provided to a wafer stage which supports the wafer. The reticle is positioned on the focus of the projection optical system by adjusting the positional relationship between the projection optical system and a reference member on a reticle stage which holds the reticle.
A first conventional example of a method of detecting the focal point of a projection optical system with respect to a reference member on a wafer stage will be described.
Referring to FIG. 7, a transmissive reticle 1 is illuminated by an illumination optical system 4, and the pattern of the reticle 1 is projected onto a resist-applied wafer 2 by a projection optical system 3. An evaluating unit 5 illuminates a focus mark which is formed on a reference member 13 on a wafer stage 12 with measuring light through a projection optical system 3, and evaluates it. The evaluating unit 5 has an illumination unit 6, a focal point changing unit 7 which changes the focal point of the evaluating unit 5 by a relay lens or the like, a photo-receiving unit 8 such as an image sensor, and the like.
A reticle stage 10 can move in a three-dimensional direction while holding the reticle 1, and is provided with a reference member 11 having a reflecting surface. A wafer stage 12 can move in the three-directional direction while holding the wafer 2, and is provided with the reference member 13 having the focus mark. A reticle stage height detector 14 measures the position (i.e., the height) of the reticle stage 10 in the direction of the optical axis of the projection optical system 3. A detector 15 which detects the height of the wafer stage 12 measures the position (i.e., the height) of the wafer stage 12 in the direction of the optical axis of the projection optical system 3. A controller 9 controls the operation of the exposure apparatus. For example, the controller 9 controls the positions of the reticle stage 10 and wafer stage 12, and detects the focal point of the projection optical system 3 while controlling the reticle stage 10, wafer stage 12, and evaluating unit 5.
The operation of the exposure apparatus shown in FIG. 7 will be briefly described. First, the exposure apparatus detects the focal point of the projection optical system 3 with respect to the reference member 13 on the wafer stage 12 in accordance with a procedure to be described later to move the wafer stage 12 to the focal point of the projection optical system 3. Subsequently, the exposure apparatus moves the reticle stage reference member 11 to the focal point of the illumination optical system 4 which is measured or adjusted in advance by using the reticle stage height detector 14, illuminates the pattern on the reticle 1 with illumination light from the illumination optical system 4, and projects and transfers the pattern onto the reticle 1 through the projection optical system 3. Before exposing the wafer 2, the exposure apparatus has an alignment measurement unit (not shown) to align the reticle 1 and wafer 2 relative to each other in a direction perpendicular to the optical axis of the projection optical system 3 as well.
The procedure for detecting the focal point of the reference member 13 on the wafer stage 12 will be described with reference to FIG. 8.
First, while moving the reticle stage 10 in the vicinity of the focus of the projection optical system 3 along the optical axis of the projection optical system 3, a contrast indicating the light quantity distribution of the focus mark on the wafer stage 12 is measured at a plurality of portions, to obtain the reticle stage position (axis of abscissa) and the contrast (axis of ordinate) obtained then, as shown in FIG. 4. The focal point of the projection optical system 3 with respect to the wafer stage 12 is calculated on the basis of the position of the reticle stage 10 at which the contrast becomes the maximum. When the reticle stage 10 is moved along the optical axis of the projection optical system 3, the reticle stage 10 is undesirably shifted from the focal point of the evaluating unit 5. Hence, the focal point of the evaluating unit 5 is changed by using the focal point changing unit 7 in the evaluating unit 5, so that the focal point of the evaluating unit 5 can be aligned with the position of the reticle stage 10.
FIG. 9 shows an example of the focus mark on the wafer stage 12. When the focus mark is at the focal point of the projection optical system 3, the contrast of the mark image of FIG. 9 becomes the maximum. As the focus mark moves away from the focal point of the projection optical system 3, the contrast decreases. For example, when the photo-receiving unit 8 includes an image sensor, the light quantity change amount in the directions (broken lines A and B in FIG. 9) of the short sides of respective rectangles in a mark image sensed by the photo-receiving unit 8 is calculated as the sum of the difference values between adjacent pixels, thereby measuring the contrast.
The focal point of the projection optical system 3 with respect to the reference member 13 on the wafer stage 12 can be specifically detected in accordance with the procedure shown in FIG. 8. First, in original driving step S201, the reticle stage 10 is moved along the optical axis of the projection optical system 3. In measured focal point changing step S202, the focal point of the evaluating unit 5 is aligned with the reference member 11 provided to the reticle stage 10 by using the focal point changing unit 7 in the evaluating unit 5. Subsequently, in light quantity measuring step S203, the focus mark image on the wafer stage 12 is sensed by using the evaluating unit 5, and the contrast of the sensed mark image is calculated by an arithmetic operation unit. This series of steps is repeatedly performed under the control of the controller 9, while changing the position of the reticle stage 10. Finally, in focal point calculating step S204, the focal point of the projection optical system 3 is calculated by the method, which is described above with reference to FIG. 4.
The positions of the reticle stage 10 and wafer stage 12 in the direction of the optical axis of the projection optical system 3 can be measured by using the reticle stage height detector 14 and wafer stage height detector 15, and controlled by the controller 9 on the basis of the measurement results.
The above method is to calculate the focal point of the projection optical system 3 with respect to the reference member 13 while changing the position of the reticle stage 10 in the direction of the optical axis of the projection optical system 3. Alternatively, the focal point of the projection optical system 3 with respect to the reference member 13 can also be calculated by measuring the contrast of the mark image while changing the position of the wafer stage 12 in the direction of the optical axis of the projection optical system 3. A contrast change, which occurs when the position of the reticle stage 10 is changed, is more moderate than a contrast change, which occurs when the position of the wafer stage 12 is changed by the square of the reduction ratio of the projection optical system 3. Therefore, the former method is advantageous in that it is less influenced by an error in position of the stage, which is measured by the optical axis method of the projection optical system 3. Generally, the reduction ratio of a projection optical system used in an exposure apparatus is 1/4 or 1/5. The influence of an error in position of the stage is accordingly, 1/16 or 1/25.
A second conventional example of the method of detecting the focal point of the projection optical system with respect to the reference member on the wafer stage will be described.
Referring to FIG. 10, a transmissive reticle 1 is illuminated by an illumination optical system 4, and the pattern of the reticle 1 is projected onto a resist-applied wafer 2 by a projection optical system 3. A measurement illumination unit 16 illuminates a focus mark which is formed on a reticle stage 10 and furthermore a focus mark formed on a wafer stage 12 through the projection optical system 3 with measuring light. The measurement illumination unit 16 has an illumination unit 6 and a focal point changing unit 7 which changes the focal point of the measurement illumination unit 16 by a relay lens or the like.
The reticle stage 10 can move in a three-dimensional direction while holding the reticle 1, and is provided with a reference member 11 having the slit-shaped focus mark. The wafer stage 12 can move in the three-directional direction while holding the wafer 2, and is provided with a reference member 13 having a slit-shaped mark and a photo-receiving unit 8, e.g., a light quantity sensor, which is arranged under the reference member 13. A controller 9 controls the operation of the exposure apparatus. For example, the controller 9 controls the positions of the reticle stage 10 and wafer stage 12, and detects the focal point of the projection optical system 3 while controlling the reticle stage 10, wafer stage 12, and measurement illumination unit 16.
In this example, the measurement illumination unit 16 and the photo-receiving unit 8 on the wafer stage 12 form an evaluating unit. A reticle stage height detector 14 measures the position (i.e., the height) of the reticle stage 10 in the direction of the optical axis of the projection optical system 3. A wafer stage height detector 15 measures the position (i.e., the height) of the wafer stage 12 in the direction of the optical axis of the projection optical system 3.
The focus marks on the reference members 11 and 13 on the reticle stage 10 and wafer stage 12 have shapes shown in FIGS. 11A and 11B, respectively. The focus marks are used for detecting the focal point of the projection optical system 3 after their positions are aligned relative to each other in a direction perpendicular to the optical axis of the projection optical system 3. When the two reference members 11 and 13 are at conjugate positions with respect to the projection optical system 3, the light quantity of the measuring light, which is to be received by the photo-receiving unit 8, from the measurement illumination unit 16, becomes the maximum. The outline of the operation of the exposure apparatus shown in FIG. 10 is identical to that of the first conventional example and a description thereof will, accordingly, be omitted.
The procedure for detecting the focal point of the reference member 13 on the wafer stage 12 will be described with reference to FIG. 6.
First, while moving the reticle stage 10 in the vicinity of the focus of the projection optical system 3 a plurality of number of times along the optical axis of the projection optical system 3, the measuring light passing through the focus marks on the reticle stage 10 and wafer stage 12 is received by the photo-receiving unit 8, and its light quantity is measured to obtain the reticle stage position (axis of abscissa) and the light quantity (axis of ordinate) as shown in FIG. 6. The focal point of the projection optical system 3 with respect to the wafer stage 12 is calculated on the basis of the position of the reticle stage 10 at which the light quantity becomes the maximum. When the reticle stage 10 is moved along the optical axis of the projection optical system 3, the reticle stage 10 is undesirably shifted from the focal point of the measurement illumination unit 16. Hence, the focal point of the measurement illumination unit 16 is changed by using the focal point changing unit 7 in the measurement illumination unit 16, so that the focal point of the measurement illumination unit 16 can be aligned with the position of the reticle stage 10.
More specifically, the focal point of the projection optical system 3 can be detected in accordance with the procedure shown in FIG. 8 in the same manner as that in the first conventional example. First, in original driving step S201, the reticle stage 10 is moved along the optical axis of the projection optical system 3. In measured focal point changing step S202, the focal point of the measurement illumination unit 16 is aligned with the reference member 11 provided to the reticle stage 10 by using the focal point changing unit 7 in the measurement illumination unit 16. Subsequently, in light quantity measuring step S203, the light quantity of the illumination light from the measurement illumination unit 16 is measured by using the photo-receiving unit 8 provided to the wafer stage 12. This series of steps is repeatedly performed under the control of the controller 9 while changing the position of the reticle stage 10. Finally, in focal point calculating step S204, the focal point of the projection optical system 3 is calculated by the method, which is described above with reference to FIG. 6.
The positions of the reticle stage 10 and wafer stage 12 in the direction of the optical axis of the projection optical system 3 can be measured by using the reticle stage height detector 14 and wafer stage height detector 15, and controlled by the controller 9 on the basis of the measurement results.
The second conventional method is to calculate the focal point of the projection optical system 3 with respect to the reference member 13 while changing the position of the reticle stage 10 in the direction of the optical axis of the projection optical system 3, in the same manner as in the first conventional example. Alternatively, the focal point of the projection optical system 3 with respect to the reference member 13 can also be calculated by measuring the illumination light quantity while changing the position of the wafer stage 12 in the direction of the optical axis of the projection optical system 3, in the same manner as in the first conventional example. Due to the same reason as in the first conventional example, the former method is advantageous in that it is less influenced by an error in position of the stage which is measured by the optical axis method of the projection optical system.
In the conventional examples described above, the focal point of a projection optical system with respect to a reference member provided to a wafer or wafer stage is detected, so that a fine circuit pattern can be exposed even in a high NA projection exposure apparatus, which has a small depth of focus.
Conventionally, as a pattern exposure method in the manufacture of a semiconductor microdevice, such as a semiconductor memory or logic circuit, reduction-projection-exposure, which uses ultraviolet rays, is used. The minimum size of a pattern that can be transferred by reduction-projection-exposure is proportional to the wavelength of light used for the transfer and inversely proportional to the NA of the projection optical system. Therefore, a decrease in wavelength of illumination light used to transfer a fine circuit pattern has been developed. The wavelength of ultraviolet light used as the illumination light is decreasing more and more, e.g., mercury lamp i-line (wavelength: 365 nm), a KrF excimer laser (wavelength: 248 nm), and an ArF excimer laser (wavelength: 193 nm). The feature size of the semiconductor device shrinks furthermore, and lithography using ultraviolet light will reach its limit sooner or later.
In order to transfer a smaller circuit pattern efficiently, a demand has arisen for a reduction projection exposure apparatus which uses extreme ultra violet light (EUV light) having a wavelength of about 10 nm to 15 nm, which is further shorter than the wavelength of ultraviolet rays, and such an apparatus is under development. EUV light is largely absorbed by a material, it is difficult to use a lens optical system, like one used with visible light or ultraviolet light, which utilizes refraction of light. Hence, in an exposure apparatus using EUV light, a reflective optical system is used. In this case, as a reticle to serve as an original, a reflecting reticle, which is obtained by forming, with an absorber, a pattern to be transferred on a mirror, is used.
FIG. 12 is a view schematically showing a reduction projection exposure apparatus which uses EUV light. Referring to FIG. 12, a reflecting reticle 1 is illuminated by an illumination optical system 4, and the pattern of the reticle 1 is projected onto a resist-applied wafer 2 by a projection optical system 3. A measurement unit 5 illuminates a focus mark on a wafer stage 12 with non-exposure light, and measures it. Alternatively, a measurement illumination unit 16 illuminates a focus mark on a reticle stage 10 and, furthermore, the focus mark on the wafer stage 12 through the projection optical system 3 with non-exposure light.
The reticle stage 10 can move in a three-dimensional direction while holding the reticle 1, and is provided with a reference member 11. The wafer stage 12 can move in the three-directional direction while holding the wafer 2, and is provided with a reference member 13. A reticle stage height detector 14 measures the position (i.e., the height) of the reticle stage 10 in the direction of the optical axis of the projection optical system 3. A wafer stage height detector 15 measures the position (i.e., the height) of the wafer stage 12 in the direction of the optical axis of the projection optical system 3. A controller 9 controls the positions of the reticle stage 10 and wafer stage 12.
Since the reduction projection exposure apparatus, which utilizes EUV light, uses the reflecting reticle, the optical axis of the evaluating unit 5 or measurement illumination unit 16 and the optical axis of the projection optical system 3 are not parallel, unlike in the first or second conventional example. Therefore, as shown in FIG. 13, when the position of the reticle stage 10 is changed, the focus mark on the reference member 13 is undesirably shifted from the optical path of the measuring light, and cannot be evaluated by the evaluating unit 5 or a photo-receiving unit 8. This phenomenon can occur regardless of the moving direction of the reticle stage 10. The fact that the focus mark cannot be evaluated means that the focus of the projection optical system 3 cannot be detected.